I am looking to go from 17 to 20 inch rims on my 2008 silverado. I will use the tire size that chevy uses on there 20's 275-55-20. My question is has anyone on here done this. How bad was gas mileage affected if at all? I have searched on the net but really have not found the answer I am hunting. I have the extended cab with the 4.8..

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well my question is do the 4.8 silverado come from factory with 20" wheels?
and if yes I guess worst case scenario is (if you are worried about gas mileage) after you get the wheels and tires take your truck to the dealer and have it program to that specific set up and I guess everything will be ok.
Did you ask the dealer and see what they say about that kind of upgrade?
I know I know they will freak when you ask them that but you might be surprised!
try thishttp://www.gmfullsize.com/forum/show...ight=what+fits

Dealer is ready to do this. I waited till my 17 inch tires were about wore out. I bought this truck new. Dealer wanted to make this change when I bought it but I decided to wear out my 17 inch first. The dealer couldn't answer the Questions I had. I am trying to find someone that has switched to 20's to see how they like it before i switch.

You know I did the same with my 18" wheels, I waited until the tires where wore out, and got me some nice 20x9 wheels on 285/50 R20 ! and no need for a leveling kit or a lift kit with this set up!!! and then sold my stock wheels on craigslist for $700 bucks!!!!!!!!!
this is what I did: I used to have a gps in my truck that tells your speed (I guess they all do) and it was dead on with the speed on my truck's speedometer, so when I got the 20's I compared the difference on speed and it was the same, gps and truck's speedometer! 'cause the tire circumference is the same as the 18" was since I put them side by side when I first got them, so in the end I didn't take my truck to the dealer to program the truck for the new wheels ans it's been ok
Now about the gas mileage, I didn't see any difference, I'm still at 16.1 MPG average.

go to tirerack.com and look up your current tire specs for the overal diameter. Compare that figure to the overall diameter of the tires you plan to go to.
Like the above post, if the overall diameters are close, like within 1/2" you wont see any significant difference in performance.
The performance difference would be the same percentage point differential between as the overall diameters.
The figuring would be more accurate if you use the "revs per mile" figures given.

I have 07 Silverado and put new tires on it and changed sizes. Tire monitoring system popped up so I went through the process of relearning tire positions on the dash. Its not working. How do I fix this?

Ok maybe the wheels where not mounted in the same positions as they where before, and you are not the only one having problems trying to relearn the tire position, even following the instructions in the owners manual!! worst case just take it to the dealer.
But for those of you who want to do this kind of upgrade (or already did it): if you didn't put the tpms in the new wheels you will have the stupid light on the dash plus the message saying "service tpms" or something like that, well you can do what I did: make a canister out of pvc, put the tpms in it (wrap them in some foam), put a valve stem (I used a 1/2 drill bit for the stem), seal it good, pump it up to 35 psi or so and you are good to go, the light will disappear!!! but if it appears again just shake the canister and it will be ok again, just keep it handy 'cause you will have to shake it every once in a while (or at least that's what I do) I think the tpms work with the wheel motion, but I might be wrong.

It's not just about tire circumference but also has to do with "rolling weight" meaning the less your wheels weigh the easier it is for you rig to move. Also the more you push weight to the outside or tread side of your wheel the harder it is on milieage. This means that width comes into the equation also. Now add friction from increased wheel tread width and things get worse. You could lose from 1 to 3 mpg from larger wheels. You gotta pay for looking "cool".

I disagree with that because the larger the wheel the more likely it is made of lighter alloys than the stock wheel which is usually steel. The tire being narrower will probably weigh less or the same as the stock tire on the stock wheel. And I still say that mileage will be higher because with every 4 rotations of the wheel(from 17 to 20") you will cover an extra foot versus stock.
and you can take the fuse out of the TPMS system. this system will be useless when the cars are older.

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